image_picker 질문

컨테이너 내부에 이미지를 갤러리에서 선택하고 띄우고 싶은데 type 'Null' is not a subtype of type 'File'이라는 에러가 나옵니다. 구글링해봐도 비슷한 
문서가 없어서 질문드립니다.

코드
GestureDetector(
  onTap: () async {
    var picker = ImagePicker();
    var image = await picker.pickImage(
      source: ImageSource.gallery);
    if(image != null){
      setState((){
        userImage = File(image.path);
      });
    } else {
      return;
    }
  },
  child: Expanded(
    child: Container(
      height: 90,
      width: MediaQuery.of(context).size.width * 0.3,
      decoration: BoxDecoration(
        color: baseColor20,
      ),
      child: Image.file(userImage),
    ),
  ),
),


에러화면

== Exception caught by widgets library =======================================================
The following _TypeError was thrown building UploadFishingVesselImages(dirty, dependencies: [MediaQuery], state: _UploadFishingVesselImagesState#c7ce0):
type 'Null' is not a subtype of type 'File'

The relevant error-causing widget was: 
  UploadFishingVesselImages UploadFishingVesselImages:file:///Users/yohankim/StudioProjects/shipda/lib/screens/vessel_sales/sales_fishing_vessel/input_fishing_vessel_description.dart:115:26
When the exception was thrown, this was the stack: 
#0      _UploadFishingVesselImagesState.build (package:shipda/screens/vessel_sales/sales_fishing_vessel/upload_fishing_vessel_images.dart:79:47)
#1      StatefulElement.build (package:flutter/src/widgets/framework.dart:4992:27)
#2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4878:15)
#3      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5050:11)
#4      Element.rebuild (package:flutter/src/widgets/framework.dart:4604:5)
#5      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2667:19)
#6      WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:882:21)
#7      RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:378:5)
#8      SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1175:15)
#9      SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1104:9)
#10     SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:881:7)
(elided 4 frames from class _RawReceivePortImpl, class _Timer, and dart:async-patch)
====================================================================================================
Reloaded 1 of 1560 libraries in 269ms (compile: 19 ms, reload: 95 ms, reassemble: 145 ms).